1. Field of the Invention
The present invention relates to a battery pack for providing power to a computer system, and more particularly to a current sensor having a selectable gain to monitor charge and discharge states of the battery pack.
2. Description of the Related Art
Computers have often been needed in locations where conventional alternating current was not available, and rechargeable batteries have been typically used as an alternative source of power, such as nickel-based batteries and lithium ion batteries, which were capable of providing power to a portable or pen-based computer system for several hours. Several rechargeable batteries were typically grouped together to form a battery of a battery pack.
Although the battery pack may have been charged in an external charger, a power supply of the host computer system typically recharged the battery pack when alternating current was available. Since rechargeable batteries have a limited cycle life, it was desirable to maximize the life of and to obtain the maximum power from each battery during every discharge cycle.
The primary design challenge confronting the earlier nickel-based battery chargers arose from the difficulty in determining the charge level of a nickel-based battery since the terminal voltage of the nickel-based battery is substantially the same regardless of the charge level. This problem was addressed by placing the microcontroller and a memory inside the battery pack as disclosed in U.S. Pat. No. 5,315,228, issued May 24, 1994, and entitled "Battery Charge Monitor and Fuel Gauge." In this battery pack, the microcontroller recalculated the total capacity of the battery pack over time, provided a fuel gauge to continually measure the remaining charge level at any given time and measured the battery pack self-discharge during periods of non-use.
A further modification of this arrangement is disclosed in commonly owned U.S. patent application Ser. No. 08/033,821, "Battery Pack Including Static Memory and a Timer for Charge Management," filed Mar. 3, 1993. In this system, the microcontroller which monitors the battery pack was moved to the host computer system. This reduced the size and cost of the battery pack; increased the battery pack shelf-life; and provided a for calculating a self-discharge rate for the battery pack.
The current in the battery of the battery pack was monitored by the microcontroller in order to control charging of the battery, monitor the discharge rate of the battery, maintain fuel gauge data and provide supervisory functions. This current was monitored for several different charge and discharge states of the battery. For example, in order to charge the battery, the battery was first fast charged, resulting in a substantial current, and thereafter the battery was trickle charged resulting in a much smaller current. Thus, when charged, the current level in the battery was a function of the charge state.
The discharge current levels in the battery also varied according to the mode of the host computer system. If the host computer system was in a standby mode, the host computer system required little power and only a small discharge current existed in the battery. However, a much larger current existed when the computer system was fully powered up.
In order to monitor the current in the battery, the prior art typically included placing a current sensor in series with the battery in order to monitor the current for all battery states. Given the variation in current level in accordance with the above-described battery states, the prior art current sensors have not provided accurate current measurements with little power dissipation for all battery states.
A low resistance, current sensing resistor has typically been used as the current sensor which dissipated little power for large battery currents. In order to measure the low voltages across this small current sensing resistor for low battery currents, though, expensive measurement circuitry has been needed to obtain accurate current measurement. Typically, costly operational amplifier circuitry with low offset voltages were used; however, the accuracy of the current measurement was still limited by the existing state of technology.
If a larger resistance was chosen for the current sensing resistor, such a resistor would produce more accurate measurements with less expensive measurement circuitry. However, this larger resistor would dissipate excessive power for large currents, such as those currents usually present during fast charge of the battery.